Which preposition should I use here: "thinking of" or "thinking about"?

Both are equally acceptable. In some contexts, thinking of might be considered slightly more poetic than thinking about; when sending a note to my sweetie, I'd probably write "Thinking of you!" instead of "Thinking about you!" Off the top of my head, though, I can't think of any cases in which one would be correct and the other flat-out incorrect.

'Thinking of' usually means 'considering'. 'I'm thinking of a new car' would mean considering replacing your current model, whereas 'thinking about a new car' is what you do when watching the adverts, without necessarily planning to buy one. (So 'thinking about getting a new keyboard' would involve "Well, typing would be faster, but it's another cable to tangle, and it costs a bit, and..."). But it's only a connotation, not a meaning, and don't try to apply it to your sweetheart!